The influence of HLA-DRB1*15 on the relationship between microglia and neurons in multiple sclerosis normal appearing cortical grey matter.

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Cortical tissue injury is common in multiple sclerosis (MS) and associates with disability progression. We have previously shown that HLA‐DRB1*15 genotype status associates with the extent of cortical inflammatory pathology. In the current study, we sought to examine the influence of HLA‐DRB1*15 on relationships between inflammation and neurodegeneration in MS. Human post‐mortem MS cases (n = 47) and controls (n = 10) were used. Adjacent sections of motor cortex were stained for microglia (Iba1+, CD68+, TMEM119+), lymphocytes (CD3+, CD8+), GFAP+ astrocytes, and neurons (NeuN+). A subset of MS cases (n = 20) and controls (n = 7) were double‐labeled for neurofilament and glutamic acid decarboxylase 65/67 (GAD+) to assess the extent of the inhibitory synaptic loss. In MS cases, microglial protein expression positively correlated with neuron density (Iba1+: r = 0.548, p < 0.001, CD68+: r = 0.498, p = 0.001, TMEM119+ r = 0.437, p = 0.003). This finding was restricted to MS cases not carrying HLA‐DRB1*15. Evidence of a 14% reduction in inhibitory synapses in MS was detected (MS: 0.299 ± 0.006 synapses/μm2 neuronal membrane versus control: 0.348 ± 0.009 synapses/μm2 neuronal membrane, p = 0.005). Neurons expressing inhibitory synapses were 24% smaller in MS cases compared to the control (MS: 403 ± 15 μm2 versus control: 531 ± 29 μm2, p = 0.001), a finding driven by HLA‐DRB1*15+ cases (15+: 376 ± 21 μm2 vs. 15−: 432 ± 22 μm2, p = 0.018). Taken together, our results demonstrate that HLA‐DRB1*15 modulates the relationship between microglial inflammation, inhibitory synapses, and neuronal density in the MS cortex.